Turbine rotors in modern jet engines and other turbine equipment generally comprise a circular array of blades extending outwardly from a rotatable shaft, the outer ends of the each blade being formed into outer shroud portions that interfit with the shroud portions of adjacent blades to create an annulus about the blades. The shroud portions are dimensioned to interfit together along joint lines called notches such that the shroud portions provides a tight external ring about the blades.
The tool and method of the present invention was developed particularly to address the problem of measuring shroud notch wear in Pratt & Whitney JT8D engines. These engines experience wear of the 92 third stage turbine blade outer shroud notches. If undetected the wear will result in failure of one or more blades and consequent damage to the downstream turbine components. Repair costs for this type of failure can exceed $300,000.
The engine manufacturer's recommendation to avoid this type of failure is to dismantle the turbine section after a set period of service to physically measure wear on the blades. Dismantling and examining of several engines should provide guidance for the degree of wear in other engines, but experience has shown that the wear rate of the notches is random. Some engines may be in danger of failure before a shop visit is due while other engines are able to continue in service long after the scheduled service period before notch wear reaches a critical state.
At present, due to the random nature of the shroud notch wear a lot of engines are dismantled unnecessarily. Since an engine shop dismantling and examination of the third stage turbine blades can cost over $100,000, there is obviously a need for a tool or method that will allow for testing of shroud notch wear without dismantling an engine at great expense.
Prior art methods for measuring wear in turbine blades are known. For example, U.S. Pat. No. 4,741,205 to Keller teaches a method of measuring shroud wear on the integrally shrouded rotating blades of a steam turbine. This method involves pre-forming special wedge notches in the shroud portions adapted to accept a wedge member that is used to apply a wedging force to move two adjacent shroud portions apart. The distance the shroud portions are moved apart is measured and provides an indication of wear on the shroud portions. This process requires that special wedging notches be formed in the rotor blades. These wedge notches are used only for measuring shroud portion wear and are a completely different item from the standard notches found in the shroud portions of the Pratt and Whitney JT8D jet engine. Keller's method therefore involves modifying an existing shroud design. In addition, it is necessary that the turbine be completely dismantled so that the distance between shroud portions can be measured as they are wedged apart.